The fears focus on the Martin-Baker US16E ejection seat. During testing of the new Generation 3 helmet this summer, testers discovered the risk of fatal neck injury when a lighter pilot ejects during slower-speed flights, according to a source with knowledge of the program. Testers discovered the ejection snapped the necks of lighter-weight test dummies, the source said.

Until the problem is fixed, the US military services decided to restrict pilots weighing under 136 pounds from operating the plane, Defense News first reported Oct. 1.

Since the issue emerged, lawmakers have vowed to push for increased oversight of the F-35, with one congresswoman condemning the program for "malpractice." Rep. Jackie Speier, ranking member of the House Armed Services Committee's subcommittee on oversight and investigations, slammed the Pentagon for rushing tests to field the plane prematurely.

"We're seeing these flight restrictions because the F-35's ejector seats weren't tested to the level they would be on a normal aircraft, and the Pentagon rushed to field them prematurely. This is yet another example of the kind of procurement malpractice we should be avoiding," the California Democrat said in an email to Defense News last week.

Meanwhile, the chairman of the House Armed Services subcommittee on tactical air and land forces pledged to hold an oversight hearing on the issue.

"We're having an F-35 hearing scheduled for Oct. 21. I'm certain it will show up then," Rep. Mike Turner, R-Ohio, said, noting that he was not previously aware of the ejection seat concern. "I am going to have an oversight hearing on this."

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At least one F-35 pilot is affected by the weight restriction, according to Joint Program Office spokesman Joe DellaVedova, who added that the rule was announced Aug. 27. He said the issue does not affect the first and only female F-35 pilot, Lt. Col. Christina Mau, the 33rd Operations Group deputy commander, he noted.

"The bottom line is they have to get into the realm where the seat allows that weight of a pilot less than 136 pounds [to] safely eject out of the airplane," Maj. Gen. Jeffrey Harrigian, the F-35 integration office director, told Defense News last week.

"They found some areas that, particularly at slower speeds, they were concerned about, so that drove the restriction that we have right now."

The ejection seat issue is not related to the new Generation 3 helmet, built by Rockwell Collins and delivered to the JPO in August, DellaVedova said. But a source with knowledge of the program said the added weight of the new helmet compared to the Gen 2 version aggravates the ejection seat issue.

A standard ejection is a two-stage event, according to Lockheed's F-35 website. First, an explosive charge or rocket motor integrated with the seat breaches the windscreen canopy. Second, the seat and pilot are launched upward via a rail system through the opening at a jarring rate of 12-14 Gs.

In August, testers discovered that when a lighter pilot is ejecting, the Martin-Baker seat rotates forward a bit too much, according to the source. That forward motion combined with the force of the ejection proved too much for the lighter dummies, snapping their necks.

"It's that light pilot and the center of gravity of the seat," Col. Todd Canterbury, who was commander of the 33rd Fighter Wing until June, told Defense News last week. "It all has to do with getting that center of gravity kind of located within the window, we call it, for safe seat-man separation."

Canterbury, who flew F-35 software versions 1B, 2A, 3i and 2B, stressed that the weight restriction is an interim fix and the JPO is working closely with Martin-Baker and aircraft builder Lockheed Martin on a permanent solution.

The Air Force expects that industry will provide a solution to meet the requirement, according to service spokesman Lt. Col. Christopher Karns.

"We are interested in a solution that is viable for all our pilots and to ensure their safety to the maximum extent practical," Karns said. "It is vitally important to ensure the F-35 community has the safest ejection seat possible. We owe it to our warfighters."

In the meantime, very few pilots appear to be impacted by the problem. The three F-35 pilots at Hill Air Force Base, Utah, are not affected by the restriction, according to Maj. Brad Matherne, assistant director of operations for the 34th Fighter Squadron.

"To be honest, it doesn't affect us at all because all our pilots weigh above 136 pounds, and to my knowledge there is only one Air Force pilot that weighs less than that, at Eglin," Matherne said in an Oct. 1 interview.

For at least one international partner, the new weight restriction is not a concern. None of Norway's F-35 pilots beginning to train on the country's new jets at Luke Air Force Base, Arizona, are affected by the weight limit, according to Col. Jarle Nergård, program manager for operations with the Norwegian F-35 Program Office.

Since the Norwegian Air Force has even tighter weight restrictions on its current F-16 fleet, "there isn't a single fighter pilot in the Norwegian Air Force that is affected" by the 136-pound restriction, Nergård told Defense News in an email.

Nergård also said discoveries like this are to be expected in a test program, and that the F-35 is meant to accommodate a greater range of pilot body types and weights than legacy fighter aircraft.

"The incredible amount of force involved once you have an ejection means that you are playing at the limits of human tolerance," Nergård said. "As partners, we do support the interim actions by the US Air Force as they are directly affected by the issue. We all have the safety of our pilots as our No. 1 priority."

Pilot safety is the services' top concern, US officials said.

"Safety is our No. 1 concern and we want to make sure that we give the warfighter the safest ejection seat capable out there," Canterbury, now the chief of the F-35 Integration Office Operations Division, said on Tuesday. "As we discover things, we can weigh the risk of what's acceptable and what's not, and right now, until we fully understand the implication of the seat, safety is our No. 1 priority."

Martin-Baker could not be reached for comment, and Lockheed Martin referred questions to the JPO.